Optical devices having “back-end” components such as infrared imaging systems or double-pass spectrometers typically use “front-end” optics, or “foreoptics”, to image the scene to the back-end components. The foreoptics have distinct requirements associated with the back-end components. The meeting of these requirements of the foreoptics either makes possible or facilitates the operation of the back-end components.
One of the most important requirements for scanning surveillance systems such as aircraft-mounted or spacecraft-mounted sensors is a wide field of view. For certain applications, the foreoptics must have a full-angle field of view, in at least one direction, of at least 30 degrees, and preferably of at least 40 degrees. It is desirable that the foreoptics have a long back focal length in order to permit the back-end components to be positioned in a container, such as a dewar, while the foreoptics is positioned outside the container and optically communicates with the back-end components through a window in the wall of the container. It is also strongly preferred in such applications that the foreoptics have an external posterior aperture stop. That is, it is preferred that the foreoptics have an aperture stop that is positioned along the beam path between the final optical element of the foreoptics and the image plane. In infrared applications, for example, the external posterior aperture stop may be placed within the interior of the dewar at one of the cold-shields of the dewar, with the front-end optical elements placed outside of the dewar. Finally, in many applications it is desirable that the propagation of light through the foreoptics not be dependent upon the wavelength of the light, because it may be important to image different wavelength bands.
There are no optical systems that are suitable as foreoptics and meet the various requirements discussed above. Accordingly, there is a need for such an optical system. The present invention fulfills this need, and further provides related advantages.